The present invention relates to a battery, especially to a non-aqueous electrolyte secondary battery. More especially, the present invention relates to an improvement in the negative electrode thereof.
The recent years, with the rapid development of portable or cordless electric appliances, demands are growing greater for such batteries having a high energy density that can drive these appliances for a long period of time.
For these demands, lithium ion secondary batteries, for instance, are attracting much attention. The lithium ion secondary batteries can obtain a high energy density by employing Li as the negative electrode thereof. However, Li is very expensive because the resources thereof are limited mainly to seawater and rock salt that contain only a scarce percentage of Li. Therefore, there is no promising possibility of reducing the price of the Li ion secondary batteries in future even by conducting a large-scale manufacturing.
For those reasons, an investigation directed to a secondary battery having a further high capacity is briskly conducted in recent years on a non-aqueous electrolyte battery employing a metal such as Mg or Al that produces polyvalent cations such as Mg.sup.2+ or Al.sup.3+ as a negative electrode active material. For instance, in a battery system employing Mg.sup.2+, two electrons are transported by a reaction per one atom of Mg. Therefore, Mg battery has a higher energy density than the Li battery.
In addition, Mg is abundant in natural resources. For that reason, Mg takes up a very large expectation as a negative electrode material. The secondary battery system employing the metal that produces a polyvalent cation such as Mg.sup.2+ is proposed in, for instance. Japanese Unexamined Patent Publications Sho 62-211861, Hei 1-95469, Hei 4-28172, respectively.
In the field of aqueous electrolyte secondary battery employing the polyvalent cation, lead-acid storage batteries, nickel-iron secondary batteries and nickel-zinc secondary batteries have conventionally developed as a low cost secondary battery. However, these aqueous electrolyte secondary batteries are difficult to raise the cell voltage because of the decomposition voltage of water. Further, in such nickel-iron secondary batteries and nickel-zinc secondary batteries, since decomposition of water in the electrolyte and a dry up of the aqueous electrolyte occur due to a high voltage during the charging stage, there is a need for supplementing water.
In the non-aqueous electrolyte secondary batteries which employ the polyvalent cation, an insulating layer may be formed on the surface of the negative electrode. Once the insulting layer is formed, the negative electrode becomes non-active and the overpotential becomes large. Therefore, the output current characteristic, the discharge capacity, voltage and cycle life characteristic of the battery are deteriorated. For these reason, utilization rate of the Mg negative electrode has been limited to as small as 10% to 20% of the theoretical capacity.
In addition, there are number of counter ions around the polyvalent cations such as Mg.sup.2+ and they hinder the immigration of the polyvalent cations. The non-aqueous electrolyte itself which has an enough ionic conductivity is the key factor to realize the higher energy density batteries. The electrolyte solution for the Li ion secondary battery employs a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC), .gamma.-butyrolactam (.gamma.-BL) or the like in which an electrolyte salt such as LiPF.sub.6 or LiBF.sub.4 are dissolved. Such electrolyte has a considerably high ionic conductivity in the case of monovalent ion of Li.sup.+, but does not have a good conductivity in the case of the polyvalent cation such as Mg.sup.2 +.